In an automobile circuit and other similar applications, a circuit interfaces with a microprocessor and not only controls a load, such as for turning the load off and on, but also monitors the condition of the load. For example, a microprocessor can generate a command signal to a circuit for turning on a lamp. Additionally, the circuit can monitor the lamp to determine when a short occurs or when the lamp (e.g., the load) is not present. These circuits are known in the prior art and typically include a power transistor for connecting and disconnecting the load with a source of voltage. The circuits also include monitoring circuitry for sensing the circuit conditions. Some of these circuits use a microprocessor having separate input and output pins on the microprocessor chip. One pin can be used for controlling the load and the other pin can be used for monitoring the load.
The circuit disclosed in U.S. Pat. No. 5,510,950 to Bills, et al. discloses a circuit where not only the condition of a load can be controlled, but also the load can be monitored from a single controller input/output pin. The disclosed circuit uses a field effect transistor that works with the microprocessor. However, this circuit is somewhat complicated and allows a small percentage of the full voltage to be used, such as for lighting a lamp, in order to give the lamp time for its filament to warm. Thus, when a filament is warming in the lamp, the high resistance would appear as a short. This circuit provides a solution for this problem by sending a small voltage. Additionally, this circuit is used on the high side of the automotive circuit, i.e., connected directly to the battery to obtain the 12 volts. The use of a high side circuit requires more complicated circuits, as compared to a low side where a switch or other circuit would be connected to the ground and then to a switch.
Another circuit disclosed in U.S. Pat. No. 5,694,282 to Yockey is designed as a low side circuit, and uses a microprocessor with its terminal alternatively configurable as an input and an output. When the terminal is configured as an output, the microprocessor can turn the electrical load on and off via the transistor under software control. When the microprocessor load turns on, the microprocessor reconfigures the terminal as an input. The system then latches the electrical load on until a short circuit occurs or until the microprocessor turns the load off. When a short circuit occurs, the system automatically turns the load off, and the microprocessor terminal is configured as an input to detect the off load.
One of the drawbacks of this circuit is that it cannot determine if an open load exists. This circuit does protect the transistor in case there is a short circuit, but it does not monitor in case there is an open load.